The present invention relates to a multilayer recording medium for optical information, which can be used for a single recording operation and essentially comprises a support and a layer comprising an unsubstituted naphthalocyanine dye.
Optical recording media for the recording of data are well known. In these media, the information is recorded in so-called "information pits" (diameter approx. 1 .mu.m), which are present in concentric or spiral-shaped tracks. The information can be recorded at a high packing density and read by optical means.
In general, information is recorded in such a way that the energy of a laser diode, which has been focused to form a spot, is irradiated through a transparent support layer and absorbed by a layer that is present on the support layer and comprises a dye and a binder. In the process, the dye/binder layer is strongly heated for a short time, and as a result of changes in the physical state, for example, due to evaporation, melting and flow processes, the above-mentioned "information pits" are generated in the dye/binder system.
The above-described recording process produces a difference in the optical characteristics (such as reflection and transmission) of the irradiated and non-irradiated areas of the layer. The different intensities of the light reflected by the irradiated and non-irradiated areas are used in the reading process for identifying the information.
A great number of patent publications describe optical recording media for use in a single recording operation, which contain organic dyes, in particular, phthalocyanine derivatives. In the form of so-called "light-absorbing layers" these phthalocyanine derivatives exhibit a high degree of stability and they are also used in combination with light-reflecting layers, for example, comprising the metals Au, Te, and Al, as described in U.S. Pat. No. 4,241,355; U.S. Pat. No. 4,298,975; and in DE-A-34 46 418.
It is a disadvantage of such metallized, light-reflecting layers that they have a high heat conductivity which often results in a reduction of the speed of recording information on media of this kind.
Phthalocyanine derivatives in the so-called light-absorbing layers, in general, show low absorption values in the emission range of the laser diodes (.lambda.=800-840 nm) used for irradiation. Particular phthalocyanines are therefore often post-treated, in a thermal process or by means of solvent vapors, in order to shift their absorption peak further into the longer wavelength region (U.S. Pat. No. 4,529,688).
In contrast, absorption in the longer wave-length region (above 800 nm) is, as a rule, achieved, when naphthalocyanine derivatives are used. It is, however, usually necessary for these naphthalocyanine derivatives to be substituted, for example, by tertiary butyl groups attached to an aromatic ring, or they must carry longer chain organic substituents on the central atom, e.g., silicon, to ensure a sufficiently high solubility in the binder layer (U.S. Pat. No. 4,492,750; EP-A-0 188 331; EP-A-0 191 215; EP-A-0 191 970; WO 8701-076-A).
The substituted naphthalocyanine derivatives have the disadvantage that they are difficult to synthesize and must be purified. Since they are dissolved in binders before being homogeneously applied to the supports (in particular by a spin-coating process) the reflectivity of the recording media is often considerably reduced. On the other hand, too low a binder content in these layers increases the close arrangement of the naphthalocyanine molecules and, as a consequence, recrystallization is more likely to occur, which, in turn, has an adverse influence on the optical characteristics of the recording media. Due to the difficulties that are still presented by the substituted naphthalocyanine derivatives in view of their solubility, the selection of suitable solvents for the binder layers is strongly restricted, when support layers comprising plastic materials, such as polycarbonate, polymethyl methacrylate, etc., are to be used.
DE-A-36 22 590 describes recording media comprising a support carrying a usually vapor-deposited naphthalocyanine layer which may be covered by a protective layer. A pigment layer having light-reflecting properties can, in particular, be arranged between the support and the naphthalocyanine layer or on top of the naphthalocyanine layer, optionally below the protective layer, if the latter is used. However, the recording sensitivity of these recording media still needs to be improved.
Multilayer arrangements are also known from other patent publications. For example, U.S. Pat. No. 4,032,691 and U.S. Pat. No. 4,636,804 describe recording media comprising a support, a light-absorbing layer which is mainly formed of metals such as, for example, nickel, aluminum, palladium, or gold and also contains specific dyes, and a porous binder layer disposed between these two layers. This version has the disadvantage that the preparation of the porous binder layer is expensive and difficult, and that the dye systems used in the absorbing layer are not very stable and show only a relatively low absorption in the required wavelength range of laser diodes.